I'm going with China. They have a huge workforce, and have, to sum this up quite simply, pretty much invested in the debt of the U.S. Which is huge. It's a safe investment. Not only that, but Communism in China has actually worked out, unlike every other country that attempted it (we don't know what's going on in North Korea, so can't really talk about if it worked out or not).
With a huge workforce, a booming GDP, sound investments, and growing amounts of political power, China could be set to overtake the U.S. Maybe. But it's all uncertain. Everything is. Except for some things.